Device for monitoring the state of a window pane

ABSTRACT

A device for monitoring the state of a window pane includes an optical emitter which emits a light beam on to the window pane. The device also contains an optical receiver which detects light of the light beam modulated by the window pane and, as a result, generates a received signal. The emitter and the receiver are arranged at a distance from the window pane. An evaluation circuit evaluates the received signal in order to determine the state of the window pane.

BACKGROUND

The invention pertains to a device for monitoring the state of a windowpane, in particular, an automobile window pane, consisting of at leastone optical transmitter which emits a light beam onto a pane, at leastone optical receiver which receives the light of the light beammodulated by the pane and subsequently generates a reception signal, andan evaluation circuit that evaluates the reception signal to determinethe state of the pane.

The comfort and safety of a motor vehicle can be enhanced by automaticoperation of the windshield wiper system. Usually, optical rate sensorslocated directly on the inside of the pane are used for this purpose.The state of the window pane on the outside of the pane is detected bythe sensor through the pane. The monitored region in this case islimited by the size of the sensor and is less than the field of view ofthe driver.

SUMMARY

The present invention is based on the task of creating a device formonitoring the state of a window pane which can be flexibly adapted todifferent operating situations and which allows a dependabledetermination of the state of the window pane.

The task is achieved in that the transmitter and the receiver arelocated at a distance from the window pane.

Due to this distant placement of both the transmitter and the receiverfrom the pane, the advantage achieved is that the size of the monitoredregion can be selected independently of the dimensions of the device. Anadditional advantage consists in that the location of the monitoredregion can be largely user-selected and can be located, for example, inthe immediate field of view of an automobile driver. Due to thepotential for prior specification of a suitable size and location of themonitored region, the dependability of the attainable evaluation resultcan be improved significantly.

Even though the measurement can also theoretically take place duringtransmission, the receiver preferably detects the light of thetransmitter through reflection or scattering off the pane. Asufficiently large monitored region with simultaneously good intensityof the reflected light can be attained when the transmitter and thereceiver are located at a distance of about 10 to 30 cm from the pane.

Preferably the light spot projected onto the pane by the light beam hasa surface area of at least 25 cm² and preferably about 100 cm². Thegeneral state of the pane will then be represented in sufficient measureby the reflected light, and local changes in the state of the pane dueto, for example, an area of dirt or a single, large rain drop, are notproblems because of the size of the monitored region and they cannotfalsify the monitored result, as is the case for sensors which arelocated directly on the glass and thus necessarily have a smallmonitored region.

The value of the monitored result will be increased when the state ofthe window pane that is being monitored is in the region of theimmediate field of view of a driver.

Preferably, the transmitter operates in the infrared range, because thiswill prevent a driver or another person that is looking through the panefrom being adversely affected by interfering reflections. Furthermore,it is an advantage that low-cost optical-electronic components operatingin the infrared range, such as Si-photodiodes (as the receiver) andinfrared LEDs (as the transmitter) are available.

According to one preferred design format of the present invention, thedevice is composed of a number of optical transmitters. Due to severaloptical transmitters, a greater transmission power can be achieved, sothat the distance between the sensor and the pane can be increased. Thismakes possible more favorable monitoring geometries and, in addition,allows the angle between the optical axis of the transmitter andreceiver, respectively, and the pane to be selected as more flat.

Even though basically several optical receivers can be provided, onepreferred embodiment is characterized in that the device is composed ofonly one optical receiver.

In the case of several optical transmitters and one single receiver, onefavorable embodiment of the invented evaluation circuit is characterizedin that the circuit is composed of a discriminator stage which uses thereception signal to derive a first and a second reception signalaccording to the acquired modulated light from one or more first and oneor more second optical transmitters. Due to this property, atransmitter-specific and, thus, also light-spot-specific, evaluation ofthe reception signal provided by the single receiver will be possible,which in practice allows a simultaneous monitoring of different regionsof the window pane.

One simple possibility for forming the first and the second receptionsignals consists in that the first and second optical transmitter iscontrolled by a pulse signal of different phase and the discriminatorstage is composed of a phase-synchronous demodulator.

Preferably, the evaluation circuit is composed of a difference stagewhich forms a difference signal from the first and the second receptionsignals. Different states of the window pane can be recognized from thetemporal change in behavior of the difference signal. Whereas adhereddirt or even damage to the window pane will cause sudden, staticchanges, fast changes are caused by large raindrops and slow changes bysmaller raindrops. Based on the state of the window pane detected inthis manner, additional measures can be undertaken, such as, forexample, the regulation of the speed of the windshield wipers.

A particularly compact design of the invented device is attained whenthe transmitter and the receiver are located in a common module orhousing.

According to one preferred configuration of the present invention, onthe input side of the optical receiver there is a reception lens whichconcentrates the light modulated by the pane onto the receiver. Due tothis property, the sensitivity of the reception device will be increasedand thus a greater distance between the sensor and the pane will bepossible.

Due to the reception lens, a reception zone will be defined on thewindow pane. With regard to attaining the greatest measurementsensitivity, it is preferable to design the reception lens so that areception zone defined on the pane by the reception lens matches theilluminated zone formed on the pane by the light spots of the lightbeam.

Due to the optical filter located on the input side in front of theoptical receiver, interference caused by incident, secondary light canbe prevented as long as the secondary light has a different wavelengththan the transmitted light. In the case of a sensor operating in theinfrared range, visible, scattered light can thus be effectivelyexcluded as a potential source of interference.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in greater detail below based on thefollowing figures and examples.

FIG. 1 is a schematic illustration of one circuit design of oneembodiment of the present invention;

FIG. 2 is a detailed view of the signal processing state shown in FIG.1;

FIG. 3 is a first example of a transmitter/receiver device;

FIG. 4 is another example of a transmitter/receiver device;

FIG. 5 is a representation of a first monitored region with anessentially square shape;

FIG. 6 is a representation of a second monitored region with anessentially square shape;

FIG. 7 is a representation of an additional monitored region with anessentially square shape; and

FIG. 8 is a representation of a monitored region with elongated shape.

DETAILED DESCRIPTION

According to FIG. 1, a device according to this invention has aninternal voltage supply 1 which is connected by means of power supplylines 2,3 to an external power supply (not illustrated in FIG. 1), forexample, the on-board power supply of a motor vehicle. Reference number4 denotes an opening in the housing. The internal power supply 1 powerstwo switching power sources 6 and 7 via a power supply line 5 and powersa microcontroller 9 via a power supply line 8. The switching powersupplies 6 and 7 are connected electrically on the output side to afirst optical transmitter 10 and to a second optical transmitter 11,respectively. The optical transmitters 10,11 operate in the infraredrange and can be designed, for example, with III-V semiconductor LEDs.

The control of the optical transmitters 10,11 takes place by means ofthe microcontroller 9. A pulse signal with a frequency of 50 kHz, forexample, is available to a digital output 12 of the microcontroller 9.The pulse signal is sent directly to the switching input of the powersupply 7 via a control line 13, but it reaches the switched input of thepower supply 6 only after passing through a 180° inverter 14. Theswitching signal output from the inverter 14 is thus phase-shifted by180° to the switching signal in the control line 13, which means thatthe switching power supplies 6,7 are alternately switched on and off andthe optical transmitters 10,11 are thus operated alternately.

The light emitted from the transmitters 10,11 moves to a window pane(not illustrated in FIG. 1) and is reflected from the window pane (in amanner to be described below) onto a receiver 15. The receiver 15 can bedesigned with a Si-photodiode, for example.

The reception signal 16 output from the receiver 15 is amplified by anamplifier 17 and then passes through a bandpass filter 18. The outputsignal 19 of the bandpass filter 18 is sent to a signal processingcircuit 20.

The circuit design of the signal processing circuit 20 is illustrated inFIG. 2. The output signal 19 is sent to the inputs of two switches21,22, which are alternately driven by switch supply lines 23,24corresponding to the pulse signal or the inverted pulse signal. In thismanner, the signal fraction of the output signal 19 provided by thefirst transmitter 10 is available at the output of the switch 21, andthe signal fraction provided by the second transmitter 11 is availableat the output of the switch 22. These signal fractions are integrated inoutlet-connected integrators 25,26 and are sent as first and secondoutput signals 27,28, respectively, to the inverting or non-invertinginput of a differential amplifier 29. Due to the differential amplifier29, the signal difference between the first and the second output signal27 and 28 will be amplified with high sensitivity and output as ananalog difference signal 30.

Therefore, the amplitude of the difference signal 30 is a measure of thedifference between the quantities of light received from the firstoptical transmitter 10 and the second optical transmitter 11. Since thetwo optical transmitters 10,11 illuminate different regions of thewindow pane, the amplitude of the difference signal 30 represents ameasure for local differences in the reflectivity and/or scatteringbehavior of the window pane.

The difference signal 30, as illustrated in FIG. 1, is sent to anA/D-converter 31 and is converted into a digital signal 32. The digitalsignal 32 is sent to a digital input 33 of the microcontroller 9.

The digital signal 32 is evaluated by the microcontroller 9 with respectto the signal amplitude and the temporal change in signal amplitude. Thetemporal change in the signal is used for recognition of differentstates of the window pane, such as may be caused, for example, byraindrops (fast changes), fine foggy mist (slow changes) or damage andadhered dirt (sudden, static changes). The information obtained from themicrocontroller 9 is sent via a bidirectional data line 34 to a driver36, which is connected over a serial data link on line 35 to an externaldata bus (not illustrated) of the motor vehicle.

A reset of the microcontroller 9 into a defined, initial state occurswhen starting the motor vehicle by means of a control line 37 betweenthe internal power supply 1 and the microcontroller 9. The initialstate, as a rule, is a default value set by the manufacturer and can bereprogrammed by the user by means of the driver 36 and the bidirectionaldata lines 34, 35 as desired.

FIG. 3 shows a first example of a rain sensor module 50 and itspositioning with respect to a window pane 51. The module 50 features asfirst optical transmitter, an IR-LED 10 a and as second opticaltransmitter, an IR-LED 11 a. The two IR-LEDs 10 a, 11 a generateilluminating light cones 52 a and 53 a each with identical opening angleα. The opening angle α can be defined by a suitable LED lens oradditional lenses. FIG. 3 makes clear that the illuminating light cones52 a, 53 a light up different light spots 54 a, 55 a on the window pane51. The light spots 54 a, 55 a can partly overlap, as in the exampleillustrated in FIG. 3.

Furthermore, the module 50 has a reception lens 56, an optical IR-filter57 and the IR receiver 15 already described in conjunction with FIG. 1.The receiver 15 is located on the optical axis of the reception lens 56and is set at a distance from it. Due to the distance between thereceiver 15 and the reception lens 56 and due to the power of the lens56 and the shape of the lens 56, a reception zone E will be defined onthe pane 51. Only reflected light (i.e., back-scattered or reflected) inthe region of the reception zone E can be detected by the receiver 15.

The mode of operation of the module 50 is as follows: by means of thecontrol of the IR-LEDs 10 a and 11 a explained in FIG. 1, the lightspots 54 a and 55 a are illuminated alternately. The receiver 15 detectsthe scattered or reflected light coming back from the alternating twolight spots 54 a and 55 a. Then, due to the switching structuredescribed in FIG. 1, a possibly different reflection and scatteringbehavior of the pane 51 will be detected in the regions of the lightspots 54 a and 55 a and evaluated with respect to temporal changes.Since the reception zone E of the two light spots 54 a and 55 a overlap,all lighted regions of the pane 51 contribute to a signal, so that amaximum light yield and thus a maximum sensitivity will be attained.Furthermore, the measuring sensitivity is also determined by therelative size of the light spots 54 a and 55 a to each other. In thecase of light spots of identical size, a maximum sensitivity will beachieved, because in this case the signal tuning can be carried out withthe greatest possible accuracy in the phase-synchronous demodulatorformed of the switches 21, 22 and the integrators 25, 26.

In the case of a pane 51′ or 51″ located at an angle to the optical axisof the reception lens 56, basically comparable conditions are present,but with identical opening angles α of the LEDs, different sizes of thelight spots 54 a and 55 a and also different distances from the receiver15 are used. This situation can be taken into account by differentconfigurations of the LEDs 10 a, 11 a with respect to their opticsand/or lighting intensity, and also by an asymmetrical signalevaluation. Furthermore, it should be taken into account that, in thecase of an inclined pane 51′ or 51″, reflected quantities of thereflected light will be increasingly reflected out of the beam path ofthe reception lens 56, which can be mostly compensated for by asuitable, axial and asymmetrical arrangement of the LEDs and also by agreater lighting intensity.

FIG. 4 shows an example of a second module 50′ with four IR-LEDs 10 a,10 b and 11 a, 11 b. In this case, the IR-LEDs 10 a, 10 b and also theIR-LEDs 11 a, 11 b are jointly controlled according to FIG. 1. The lightspots 54 a, 54 b generated by the jointly controlled IR-LEDs 10 a, 10 btogether form a first illuminated zone I, while the light spots 55 a, 55b of the IR-LEDs 11 a, 11 b are combined into a second illuminated zoneII. A different reflection or scattering behavior of the pane 51 ismeasured in the illuminated zones I and II. In this case, the receptionzone E′ is located within the total lighting region formed by the twoilluminated zones I and II.

Based on the increase in the number of IR-LEDs 10 a, 10 b, 11 a, 11 b incomparison to the first module 50 shown in FIG. 3, a greater distancebetween the rain sensor module 50′ and the pane 51 is possible, andfurthermore, a flatter angle can be used between the optical axis of thelens 56 and the pane 51′, 51″.

FIGS. 5,6,7 and 8 show various possibilities for lighting of the pane51, and also the selection of the reception zone E.

According to FIG. 5, four light spots form intersecting, diagonallyarranged illuminated zones I and II. The reception zone E overlaps theoverall illuminated zone I and II well.

According to FIG. 6, two illuminated zones I and II located above eachother are formed by four light spots. Here, too, there is a good overlapbetween the overall illuminated zone I and II anal the reception zone E.

The geometries shown in FIGS. 5 and 6 can be produced with the module50′ described in FIG. 4.

FIG. 7 shows illuminated zones I and II, which are formed from a totalof eight light spots. The illuminated zones I and II are arranged in adiagonal cross, similar to FIG. 5. The reception zone E′ is configuredin the shape of a circular disk as in FIGS. 5 and 6.

In FIG. 8, the illuminated zones I and II are formed from six lightspots located side by side, and neighboring light spots are allocated todifferent illuminated zones I or II. Here, the elliptical orcigar-shaped reception zone E″ nearly circumscribes the entireilluminated surface and can be attained in a suitable manner by acombination of a spherical lens with a cylindrical lens, or by anastigmatic lens. The lighting geometry shown in FIG. 8 corresponds tothe field of view of a driver of a motor vehicle, wherein the measuredvalues can be ascertained in a highly accurate manner and thus controlof the windshield wipers can be obtained.

In the case of abrasion, such as scratches on the window pane, the twoareas detected by the sensor will not be identical. Therefore, in thiscase the transmitter regions I and II will not be alternated at asensing ratio of 50:50, but rather of an asymmetrical sensing ratio of,e.g., 70:30. Due to the use of an integrated phase synchronousdemodulator, this asymmetrical behavior can be taken into account.

Due to this variable sensing ratio, the sensor will thus compensate forfaults, dirt and other static events. The sensor will recognize a changeas static when it exceeds a characteristic time constant. lens, or by anastigmatic lens. The lighting geometry shown in FIG. 8 corresponds tothe field of view of a driver of a motor vehicle, wherein the measuredvalues can be ascertained in a highly accurate manner and thus a controlof the windshield wipers can be obtained.

In the case of abrasion such as scratches on the window pane, the twoareas detected by the sensor will not be identical. Therefore, in thiscase the transmitter regions I and II will not be alternated at asensing ratio of 50:50, but rather of an asymmetrical sensing ratio of,e.g., 70:30. Due to the use of an integrated phase synchronousdemodulator, this asymmetrical behavior can be taken into account.

Due to this variable sensing ratio, the sensor will thus compensate forfaults, dirt and other static events. The sensor will recognize a changeas static when it exceeds a characteristic time constant.

What is claimed is:
 1. A device for monitoring the state of a windowpane comprising a first optical transmitter which emits a first lightbeam onto a pane to illuminate a first region of the pane, a secondoptical transmitter which emits a second light beam onto a pane toilluminate a second region of the pane, at least one optical receiverwhich receives the light of the first and second light beams modulatedby the first and second regions of the pane, respectively, andsubsequently generates a reception signal, and an evaluation circuitthat evaluates the reception signal to determine the state of the paneat the first and second regions of the pane, characterized in that thetransmitters and the receiver are located at a distance from the paneand the evaluation circuit is composed of a discriminator stage whichuses the reception signal to derive a first and a second receptionsignal according to the detected modulated light from at least one firstoptical transmitter and at least one second optical transmitter,respectively.
 2. The device according to claim 1 characterized in thatthe light received by the receiver includes reflected or scattered lightfrom the light beams.
 3. The device according to claim 1 characterizedin that the transmitters and the receiver are located at a distance ofabout 10 to 30 cm away from the pane.
 4. The device according claim 1characterized in that a light spot projected from a light beam onto thepane has a surface area of at least 25 cm².
 5. The device according toclaim 1 characterized in that the window pane is an automobile windowpane and the state of the window pane is monitored in the region of theimmediate field of view of a driver of the automobile.
 6. The deviceaccording to claim 1 characterized in that the transmitter operates inthe infrared range.
 7. The device according to claim 1 characterized inthat the device includes a single optical receiver which receives thelight modulated by the pane from the light beams emitted from theoptical transmitters.
 8. A device for monitoring the state of a windowpane including at least one optical transmitter which emits a light beamonto a pane, at least one optical receiver which receives the light ofthe light beam modulated by the pane and subsequently generates areception signal, and an evaluation circuit that evaluates the receptionsignal to determine the state of the pane, characterized in that the atleast one optical transmitter and the receiver are located at a distancefrom the pane, and the device is composed of a plurality of opticaltransmitters, and the evaluation circuit is composed of a discriminatorstage which uses the reception signal to derive a first and a secondreception signal according to the detected modulated light from at leastone first optical transmitter and at least one second opticaltransmitter, wherein the first optical transmitters and second opticaltransmitters are controlled by a pulse signal of a different phase andthe discriminator stage is formed of a phase-synchronous demodulator. 9.The device according to claim 1 characterized in that the evaluationcircuit is composed of a difference stage which forms a differencesignal from the first reception signal and the second reception signal.10. The device according to claim 9 characterized in that the evaluationcircuit is composed of a microcontroller that is supplied with thedifference signal and that evaluates a temporal change in behavior ofthe difference signal.
 11. The device according to claim 1 characterizedby at least four optical transmitters whose light beams illuminate anessentially rectangular illuminated zone on the pane formed by partiallyoverlapping light spots.
 12. The device according to claim 1characterized by at least three optical transmitters whose light beamsilluminate an essentially elongated illuminated zone on the pane formedby partially overlapping light spots.
 13. The device according to claim1 characterized in that the transmitters and receiver are housed in acommon housing.
 14. The device according to claim 1 characterized inthat on the input side of the optical receiver there is a reception lenswhich concentrates the light modulated by the pane onto the receiver.15. The device according to claim 14 characterized in that the receptionlens is designed so that a reception zone defined by the reception lenson the pane is adapted to the illuminated zone formed on the pane by thelight spots of the light beam.
 16. The device according to claim 14characterized in that the reception lens is one of a spherical lens, acylindrical lens and an astigmatic lens.
 17. The device according toclaim 1 characterized in that there is an optical filter on the inputside of the optical receiver.
 18. The device according to claim 1characterized in that the at least one first optical transmitter and theat least one second optical transmitter are controlled by a pulse signalof a different phase and the discriminator stage is formed of aphase-synchronous demodulator.
 19. The device according to claim 8characterized by at least four optical transmitters whose light beamsilluminate an essentially rectangular illuminated zone on the paneformed by partially overlapping light spots.
 20. The device according toclaim 8 characterized by at least three optical transmitters whose lightbeams illuminate an essentially elongated illuminated zone on the paneformed by partially overlapping light spots.